A condenser microphone unit is known that has a diaphragm vibrating in response to received sound waves that faces a fixed electrode across a spacer with a gap corresponding to the thickness of the spacer. A condenser microphone is also known that includes the condenser microphone unit. The diaphragm and the fixed electrode function as electrodes of a capacitor. The diaphragm vibrates in response to received sound waves to change the gap distance between the diaphragm and the fixed electrode. This varies the capacitance between the diaphragm and the fixed electrode. The variation in the capacitance responds to the sound waves and can be outputted as audio signals through electroacoustic conversion.
Condenser microphone units have a disadvantage of a large variation in sensitivity, i.e., output level. The variation in the sensitivity is caused by, for example, a variation in a voltage across the electrodes, i.e., between the diaphragm and the fixed electrode, a variation in the gap distance between the diaphragm and the fixed electrode, and distortion of the diaphragm and the fixed electrode. In particular, electret condenser microphone units, in which either the diaphragm or the fixed electrode is composed of an electret material to semipermanently store charges, have a large variation in the charges. This causes a large variation in a voltage between electrodes composed of the diaphragm and the fixed electrode, resulting in a large variation in the sensitivity.
Meanwhile, condenser microphone units output significantly low levels of audio signals through electroacoustic conversion. Thus, to enhance the output level of a condenser microphone unit, the inventors applied for a patent on a condenser microphone including multiple condenser microphone units in which diaphragms are arranged on the same plane and the condenser microphone units are connected in series such that outputs from an impedance converter connected to one condenser microphone unit drive the ground terminal of another condenser microphone unit (see Japanese Patent Laid-Open Publication No. 2011-10046).
The condenser microphone described in Japanese Patent Laid-Open Publication No. 2011-10046 can accumulate outputs of the condenser microphone units connected in series to enhance the output level of the microphone. In addition, a first group composed of some of the condenser microphone units connected in series outputs hot signals of a balanced output while a second group composed of the other condenser microphone units connected in series outputs cold signals of the balanced output. This can further double the output level of the microphone. The outputs of individual microphone units can be accumulated as described above to provide a supersensitive condenser microphone and relatively reduce the equivalent sound pressure level of noise.
It is preferred that a difference in sensitivity should be eliminated among microphone units used in order to enhance the sensitivity in a condenser microphone described in, for example, Japanese Patent Laid-Open Publication No. 2011-10046. Even if the diaphragms of the microphone units are arranged on the same plane to align apparent directive axes, the microphone units with different sensitivities have directive axes misaligned with the directive axis of the microphone, and thereby the microphone cannot have an aligned directive axis. A difference in sensitivity should therefore be eliminated among the microphone units to equalize levels of audio signals outputted from the individual microphone units.
It is however significantly difficult to reduce the difference in sensitivity within, for example, ±1 dB among individual condenser microphone units of a conventional condenser microphone due to the factors causing such a difference as described above. A condenser microphone unit having adjustable sensitivity would be useful. A condenser microphone unit having sensitivity adjustable after being assembled in a microphone case is more useful. In particular, in a condenser microphone that accumulates output signals from multiple microphone units, adjustment of the sensitivities of individual microphone units is advantageous.